Ultrasonic, flow measuring devices are widely applied in process and automation technology. They enable convenient determination of volume flow and/or mass flow in a pipeline.
Known ultrasonic, flow measuring devices work frequently according to the travel-time difference principle. In the case of the travel-time difference principle, the different travel times of ultrasonic waves, especially ultrasonic pulses, so-called bursts, are evaluated relative to the flow direction of the liquid. For this, ultrasonic pulses are sent, i.e. transmitted, at a certain angle to the tube axis both in the direction of the flow as well as also counter to the flow. From the travel-time difference, the flow velocity, and therewith, in the case of known diameter of the pipeline section, the volume flow, can be determined.
The ultrasonic waves are produced and received with the assistance of so-called ultrasonic transducers. For this, ultrasonic transducers are fixed in the tube wall of the relevant pipeline section. Also clamp-on ultrasonic, flow measuring systems are obtainable. In the case of clamp-on systems, the ultrasonic transducers are pressed externally of the measuring tube on its tube wall. A great advantage of clamp-on ultrasonic, flow measuring systems is that they do not contact the measured medium and are placed on an already existing pipeline.
The ultrasonic transducers are normally composed of an electromechanical transducer element, e.g. a piezoelectric element, and a coupling layer. In the electromechanical transducer element, the ultrasonic waves are produced as acoustic signals and led via the coupling layer to the tube wall and from there into the liquid in the case of clamp-on systems. In the case of inline systems, the ultrasonic waves pass via the coupling layer directly into the measured medium. In such case, the coupling layer is also (not so frequently) referred to as a membrane.
Between the piezoelectric element and the coupling layer, another coupling layer can be arranged, a so called adapting, or matching, layer. The adapting, or matching, layer performs, in such case, the function of transmitting the ultrasonic signal and simultaneously reducing a reflection caused by different acoustic impedances at boundaries between two materials.
Both in the case of clamp-on systems as well as also in the case of inline systems, the ultrasonic transducers are arranged on the measuring tube in a shared plane, either on oppositely lying sides of the measuring tube, in which case the acoustic signal moves, projected on a tube cross section, once along a secant through the measuring tube, or on the same side of the measuring tube, in which case the acoustic signal is reflected on the oppositely lying side of the measuring tube, whereby the acoustic signal traverses the measuring tube twice along secants projected on the cross section through the measuring tube. U.S. Pat. Nos. 4,103,551 and 4,610,167 show ultrasonic, flow measuring devices with reflections on reflection surfaces provided in the measuring tube. Also multipath systems are known, which have a number of ultrasonic transducer pairs, which, in each case, form a signal path, along which the acoustic signals extend through the measuring tube. The respective signal paths and the associated ultrasonic transducers lie, in such case, in planes mutually parallel and parallel to the measuring tube axis. U.S. Pat. Nos. 4,024,760 and 7,706,986 show, by way of example, such multipath systems. An advantage of multipath systems is that they can measure the profile of the flow of the measured medium in the measuring tube at a number of locations and thereby provide highly accurate measured values for the flow. This is achieved based on, among other things, also the fact that the individual travel times along the different signal paths are differently weighted. Disadvantageous in the case of multipath systems is, however, their manufacturing costs, since a greater number of ultrasonic transducers and, in given cases, a complex evaluating electronics are applied.
There are different approaches for weighting the signal paths. The paper “Comparison of integration methods for multipath acoustic discharge measurements” by T. Tresch, T. Staubli and P. Gruber in the Proceedings of the 6th International Conference on Innovation in Hydraulic Efficiency Measurements, 30 Jul.-1 Aug. 2006 in Portland, Oreg., USA, compares established methods for weighting the travel times along different signal paths for calculating flow.
European patent, EP 0 715 155 A1 proposes a measuring arrangement utilizing multiple refraction, wherein the subsections of the signal path form only one plane, which extends parallel to the measuring tube axis. The reflection surfaces on which a first subsection of the signal path ends and a second subsection of the signal path begins are shown in EP 0 715 155 A1 as planar bodies, which are placed on the inner side of the tube. It is, indeed, theoretically possible, to introduce reflection surfaces from the ends of a measuring tube and then to weld them to the inner wall of the measuring tube. However, such a manufacture in the case of smaller measuring tubes with small nominal diameters rapidly approaches its limits, since a welding device can in the case of small nominal diameters only be applied with great effort and is accompanied by loss of precision as regards the positioning of the reflection unit. Thus, the teaching of EP 0 715 155 A1 is for measuring tubes with large nominal diameters.
German patent, DE 10 2008 055 030 A1 describes a connection nozzle formed by hydroforming in an ultrasonic flow measurement device. An ultrasonic transducer is inserted into the connection nozzle. Signal transmission occurs along a straight signal path without reflection on the tube wall. The measuring tube of the flow measuring device has, in such case, a flat shape, so that—in other than in the case of round cross sections—in the case of this measuring tube, less disturbances in the flow profile from vortices can occur.
German patent, DE 102 49 542 A1 describes a coupling surface for in-coupling of an ultrasonic signal from an ultrasonic transducer into a measuring tube, wherein the coupling surface is formed inclined to the measuring tube. The measuring tube includes additionally a formed body 10, which provides a reflection surface.
European patent, EP 0 303 255 A1 describes a measuring tube of an ultrasonic flow measuring device, in which a reflection surface is embodied integrally with the measuring tube. In such case, a cross-sectional widening of the measuring tube occurs over a broad region, and this is unfavorable for accuracy of the measurement data.
German patent, DE 10 2012 013 916 A1 shows, in contrast, a measuring tube of an ultrasonic flow measuring device with screwed-in reflection surfaces. In such case, first of all, a connection nozzle with a screw thread is formed, into which a reflector can then be inserted. This manufacturing variant has basically been successful for all measuring tubes, independently of their nominal diameter. This manufacture requires, however, the exact maintaining of predetermined bore patterns and each connection nozzle must be separately processed before the insertion of the reflector.
An alternative, already known variant is the casting of the tube and the welding of nozzles onto the measuring tube and the following screwing in or welding on of a reflection surface.
Known from German patent, DE 10 2013 105 922 A1 is an ultrasonic, flow measuring device with a measuring tube produced by means of a high pressure forming method. The geometric accuracy and the orientation of the area normals at the desired angle is, however, less in the case of formed reflection surfaces compared with screwed reflectors.